U.S. Pat. No. 3,721,778 describes a keyboard switch which has its own internal logic to provide an encoded output. The output is achieved without the necessity of using logic devices external to the switch as is required, for example, in a conventional matrix encoded keyboard switch. The keyboard switch described in U.S. Pat. No. 3,721,778 is sound in principle in that it permits the elimination of solid state circuit devices which are otherwise required to interpret the output of most keyboard switches. The physical structure of the keyboard described in U.S. Pat. No. 3,721,778 is, however, relatively complex and a switch as described in that U.S patent involves some difficult manufacturing and assembly operations. The present invention, in accordance with one aspect thereof, is directed to the achievement of a keyboard switch having its own internal logic for encoding which can be manufactured with relative ease by commonly understood and easily executed switch manufacturing processes.
It is important in keyboard switches that the signals which are transmitted from the switch to be valid signals which accurately convey the intelligence or information which was intended by the person operating the switch. Problems can arise, under some circumstances, if the person operating the switch does not carefully depress the key tops or the keys of a membrane switch in that inaccurate signals may be produced as a result of the fact that the circuits which are intended to be energized are not energized. In other words, if the operator only partially depresses the key top of a keyboard switch and only some of the necessary circuits are energized, information may be transmitted by the switch which is faulty, in that it is not the same information which the operator intended to transmit.
The present invention in accordance with a further aspect thereof, is directed to the achievement of a keyboard switch which has internal circuitry that ensures that any signal or information produced in the switch accurately reflects the intention of the operator of the keyboard switch.
An encoded keyboard switch assembly in accordance with the invention has a plurality of key positions, each key position having a key top and a first associated membrane switch immediately beneath the key top. Depression of the key top thus closes the first associated membrane switch immediately. Each first associated membrane switch has first and second switch contacts, the first switch contacts being commonly connected to a first common external conductor extending from the switch assembly. The second switch contacts have output conductors extending therefrom to external signal conductors. The second switch contacts are selectively connected to each other by encoding conductors in accordance with an encoding scheme, such as a hexadecimal encoding scheme. Each of the key positions also has a second associated membrane switch which is beneath the first associated membrane switch. Each second associated membrane switch has third and fourth switch contacts. The third contacts are commonly connected to a second common external conductor and the fourth contacts are commonly connected to an internal conductor, which is in turn commonly connected to the first contacts of the first associated membrane switches. The arrangement is such that information cannot be sent through the signal conductors until the second associated membrane switch contacts are closed. The information sent is therefore an accurate transmission of the information intended by the operator who depresses the key top.